While Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, single-site (metallocene and non-metallocene) catalysts represent the industry's future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of α-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Single-site olefin polymerization catalysts having “open architecture” are generally known. Examples include the so-called “constrained geometry” catalysts developed by scientists at Dow Chemical Company (see, e.g., U.S. Pat. No. 5,064,802), which have been used to produce a variety of polyolefins. “Open architecture” catalysts differ structurally from ordinary bridged metallocenes, which have a bridged pair of pi-electron donors. In open architecture catalysts, only one group of the bridged ligand donates pi electrons to the metal; the other group is sigma bonded to the metal. An advantage of this type of bridging is thought to be a more open or exposed locus for olefin complexation and chain propagation when the complex becomes catalytically active. Simple examples of complexes with open architecture are tert-butylamido(cyclopentadienyl)dimethyl-silylzirconium dichloride and methylamido(cyclopentadienyl)-1,2-ethanediyltitanium dimethyl: 
Organometallic complexes that incorporate “indenoindolyl” ligands are known (see U.S. Pat. No. 6,232,260 and PCT Int. Appl. WO 99/24446 (“Nifant'ev”)). The '260 patent demonstrates the use of non-bridged bis(indenoindolyl) complexes for making HDPE in a slurry polymerization. Versatility is an advantage of the complexes; by modifying the starting materials, a wide variety of indenoindolyl complexes can be prepared. “Open architecture” complexes are neither prepared nor specifically discussed. Nifant'ev teaches the use of bridged indenoindolyl complexes as catalysts for making polyolefins, including polypropylene, HDPE and LLDPE. The complexes disclosed by Nifant'ev do not have open architecture.
PCT Int. Appl. WO 01/53360 (Resconi et al.) discloses bridged indenoindolyl complexes having open architecture and their use to produce substantially amorphous propylene-based polymers. Resconi teaches many open architecture complexes but none that are bridged through the indolyl nitrogen. Moreover, all of the complexes are used only to make propylene polymers; their use to produce low-density ethylene polymers is not disclosed. Resconi's teachings are also limited to indeno[2,1-b]indolyl complexes; the reference includes no disclosure of indeno[1,2-b]indolyl complexes or their use for making propylene polymers.
As noted earlier, the indenoindolyl framework is versatile. The need continues, however, for new ways to make polyolefins—especially ethylene copolymers—with very low densities. In particular, it is difficult to make ethylene polymers having densities less than about 0.915 g/cm3 using known indenoindolyl complexes. On the other hand, ethylene polymers having such low densities are valuable for special applications requiring elastomeric properties. The industry would also benefit from the availability of new catalysts that capitalize on the inherent flexibility of the indenoindolyl framework.